A digital watermark, embedded in a digital (possibly multimedia) document, is a sequence of information typically for identifying the owner of the document. The information can thus comprise some unique identifier of the owner or copyright information pertinent to the document and its owner. It is commonly an invisible mark inserted into digital multimedia data that can be detected at some later time for evidence of rightful ownership, whether of the original document or copyright therein.
Techniques proposed so far can be divided into two main groups, according to the embedding domain of the container image (Miller et al. 1998). One is the spatial domain approach. The earliest digital watermarking techniques are mainly this kind and the simplest example is to embed the watermark into least significant bits (LSBs) of the image pixels (van Schyndel et al. 1994). However, this technique has relatively low information hiding capacity and can be easily erased by lossy image compression.
Another is the frequency domain approach, in which more information bits can be embedded and which is relatively robust to attacks.
Also, spread spectrum communication has been used for digital multimedia watermarking (Cox et al. 1997); in this approach, a Gaussian distributed sequence was embedded into the perceptually most significant frequency components of a container image.
In another approach (Hsu and Wu 1999), an image watermark was embedded into selectively modified middle frequencies of discrete cosine transform (DCT) coefficients of a container image. Another existing approach (Joseph et al. 1998) comprises a digital image watermarking using the Fourier-Mellin transform that is invariant to image manipulations or attacks due to rotation, scaling and translation. Several other methods (Wei et al. 1998, Dugad et al. 1998, Hsu and Wu 1998) used discrete wavelet transform (DWT) to hide the data to the frequency domain, including (Wei et al. 1998) applying JND (just-noticeable distortion) feature of HVS (human visual system) in wavelet transform domain and hiding 236 information bits in the ‘lenna’ image.
Prior art techniques, however, rely on the use of a database containing some template against which the image is compared. For example, those existing techniques that incorporate an invisible mark (representing, for example, an identification number) store the identification number in a database. During any verification process, the number in the database is retrieved and effectively compared with the number that has been embedded within the watermarked document. Such systems, however, have a number of disadvantages. Firstly, of course, a database must be provided and maintained, thereby introducing added cost. Further, the database itself adds a security risk. Systems that are designed around the use of a database generally assume that the identification number is an important key that must be kept secure; reliance on a database actually creates an additional point of vulnerability (i.e. the database itself) and costly security measures must then be employed to guard the integrity of the database against those who would forge documents.